In vitro nucleic acid amplification techniques have provided powerful tools for detection and analysis of small amounts of nucleic acids. The extreme sensitivity of such methods has lead to attempts to develop them for diagnosis of infectious and genetic diseases, isolation of genes for analysis, and detection of specific nucleic acids as in forensic medicine.
In general, diagnosis and screening for specific nucleic acids using nucleic acid amplification techniques has been limited by the necessity of amplifying a single target sequence at a time. In instances where any of multiple possible nucleic acid sequences may be present (e.g., infectious disease diagnosis), performing multiple separate assays by this procedure is cumbersome and time-consuming. U.S. Pat. Nos. 4,683,195; 4,683,202 and 4,800,159 describe the PCR. Although these inventors state that multiple sequences may be detected, no procedure for amplifying multiple target sequences simultaneously is disclosed. When multiple target sequences are amplified, it is by sequentially amplifying single targets in separate PCRs. In fact, when multiple pairs of primers directed to different target sequences are added to a single PCR, the reaction produces unacceptably high levels of nonspecific amplification and background. An improvement on the PCR which reportedly allows simultaneous amplification of multiple target sequences is described in published European Patent Application No. 0 364 255. This is referred to as multiplex DNA amplification. In this method, multiple pairs of primers are added to the nucleic acid containing the target sequences. Each primer pair hybridizes to a different selected target sequence, which is subsequently amplified in a temperature-cycling reaction similar to PCR. Adaptation of PCR to footprinting is taught by P. R. Mueller and B. Wold (1989. Science 246, 780-786). For footprinting, a common oligonucleotide sequence is ligated to the unique end of each fragment of the footprint ladder. The fragments are simultaneously amplified using a primer complementary to the common sequence and a primer complementary to the known sequence of the fixed end.
In most cases, nucleic acid amplification techniques have been used to produce qualitative results in diagnostic assays. However, there has been great interest in developing methods for nucleic acid amplification which are not only capable of detecting the presence or absence of a target sequence, but which can also quantitate the amount of target sequence initially present. Internal control sequences have been used in the PCR in an attempt to produce such quantitative results. Parent application U.S. Ser. No. 08/058,648 discloses internal controls sequences useful in isothermal nucleic acid amplification reactions for quantitating target sequence as well as determining the amplification activity of the sample (i.e., efficacy--whether or not the sample inhibits the amplification reaction, thus producing a false negative result).
Certain PCRs which employ internal controls select internal control sequences which can be amplified by the same primers as the target sequence. See, for example, WO 93/02215 and WO 92/11273. In the PCR, the amplified target and control sequences may distinguished by different fragment lengths as the rate of the PCR is known to be relatively unaffected by the length of the target and does not significantly affect amplification efficiency. EP 0 525 882 describes a method for quantifying a target nucleic acid in a Nucleic Acid Sequence Based Amplification (NASBA) reaction by competitive amplification of the target nucleic acid and a mutant sequence. The method is performed with a fixed amount of sample and a dilution series of mutant sequence. The analysis involves determining the amount of added mutant sequence which reduces the signal from the target sequence by 50%, i.e., the point at which the mutant sequence and target sequence are present in equal amounts. To produce accurate quantification, the amplification reactions described in EP 0 525 882 must be allowed to continue until at least one of the reagents is essentially exhausted, i.e., into the post-exponential phase of the reaction where competition for limited reagents can occur. Furthermore, the calculations are accurate only when two reactions are competing for reagents--the target amplification and the mutant sequence amplification. The results are therefore not reliable when a third reaction, such as background amplification, is occurring. As essentially all amplification reactions include some degree of background amplification, the EP 0 525 882 quantifying method is only accurate for a high level of target sequence. At low target levels, competing background amplification reactions would significantly interfere with the accuracy of the calculations. Because it relies on amplifying various dilutions of the mutant sequence with the target, the EP 0 525 882 method is also susceptible to tube-to-tube variations in the amount of mutant and target sequence. Even small differences in the amount of target sequence or slight inaccuracies in the dilutions of mutant sequence between tubes are exponentially amplified in the subsequent amplification reaction and are reflected in the quantification calculations.
In contrast, the method of U.S. Ser. No. 08/058,648 does not require competition between control and target sequences for reagents nor does it require that the reaction go into the post-exponential phase. It is accurate in both the exponential and post-exponential phases of the amplification reaction. The ratio of target/control sequence is therefore not adversely affected by background amplification reactions which may be occurring and remains the same regardless of the extent of background reaction. The result can therefore be obtained earlier in the amplification reaction and variability is reduced by the use of a single target/control coamplification reaction rather than a series of reactions.
Previously reported multiplex nucleic acid amplification methods require a separate internal control sequence matched to each target to be amplified because each target is amplified using a different pair of primers (i.e., a control sequence for each primer pair). Prior to the present invention it was not possible to use a single internal control sequence to monitor or quantitate multiple targets in multiplex nucleic acid amplification reactions. It is therefore a feature of the instant adapter-mediated multiplex amplification methods that the single pair of primers required for multiplex amplification makes it possible for the first time to use a single internal control sequence to monitor or quantitate amplification of the multiple targets.
The Mycobacteria are a genus of bacteria which are acid-fast, non-motile, gram-positive rods. The genus comprises several species which include, but are not limited to, Mycobacterium africanum, M. avium, M. bovis, M. bovis-BCG, M. chelonae, M. fortuitum, M. gordonae, M. intracellulare, M. kansasii, M. microti, M. scrofulaceum, M. paratuberculosis and M. tuberculosis. Certain of these organisms are the causative agents of disease. For the first time since 1953, cases of mycobacterial infections are increasing in the United States. Of particular concern is tuberculosis, the etiological agent of which is M. tuberculosis. Many of these new cases are related to the AIDS epidemic, which provides an immune compromised population which is particularly susceptible to infection by Mycobacteria. Other mycobacterial infections are also increasing as a result of the increase in available immune compromised patients. Mycobacterium avium, Mycobacterium kansasii and other non-tuberculosis mycobacteria are found as opportunistic pathogens in HIV infected and other immune compromised patients.
At the present time the diagnosis of mycobacterial infections is dependent on acid-fast staining and cultivation of the organism, followed by biochemical assays. These procedures are time-consuming, and a typical diagnosis using conventional culture methods can take as long as six weeks. Automated culturing systems such as the BACTEC.TM. system (Becton Dickinson Microbiology Systems, Sparks, Md.) can decrease the time for diagnosis to one to two weeks. However, there is still a need to reduce the time required for diagnosing Mycobacterial infections to less than a week, preferably to about one day. Oligonucleotide probe based assays such as Southern hybridizations or dot blots are capable of returning a rapid result (i.e., in one day or less). Assays based on amplification of nucleic adds may provide even more rapid results, often within hours. For diagnosis of Mycobacterial infections such methods would require an oligonucleotide probe or primer which is specific for the genus of Mycobacteria or specific for a particular mycobacterial species if specific identification of the organism is desired.